In recent years, there has been a major expansion in the technology of information storage on recordable media. Principal such media are magnetic tapes and optically readable discs and film. Although magnetic tape recording and more recently optical data storage techniques have been around for some time, it is only in relatively recent years that concern has arisen with respect to the long-term stability of such media during storage which can adversely affect the integrity and readability of the data. This arises in some cases from long-term slow degradation of the polymeric material constituting the tape backing or the matrix in which optically readable bits have been impressed or otherwise provided. In some cases, other components of the recording medium may constitute a source of degradation themselves, but it is widely reckoned that in all cases, a major problem source is interaction with the environment, both with oxygen in the atmosphere and with other components which, although they may be present in very small concentrations, have an effect over long time scales. One example of this is moisture or water vapour.
The classical approach to preserving recorded media, for example adopted widely in the film industry, is to store reels on which the media is wound in tins or similar canisters. These are not airtight, and the atmosphere within the tin corresponds to the ambient atmosphere when it is closed, and thus contains oxygen moisture, pollutants, air borne particulates and proportions of sundry other ambient components. Relative airtightness can be provided by, e.g. placing an adhesive tape around the line between the edge of the lid and the base, but this does not prevent deterioration.
It is known to preserve the properties of internally pressurised balls by storing them in a container which is itself pressurised. The time for which the balls need to be kept under pressure is relatively small, weeks or months, and the containers are not designed for re-use or long-term archival storage. Examples of pressurised tennis ball containers are known from WO93/20901, DE-A-3805008, DE-A-4001610 and DE-A-4106374.
It is also known to preserve ultra-sensitive materials, particularly semiconductor wafers, from deterioration arising from external influences by locating them in a sealed container and placing them under partial vacuum surrounded by an inert atmosphere, see GB-A-2262513. Such containers are not designed for long-term archival storage, but rather merely to enable wafers to be transported without contamination from one processing step to another.
According to a first feature of the present invention, there is provided a method of long-term storage of recorded or recordable media which comprises placing the media in a sealable container, purging the interior of the container of atmosphere by introducing into the interior of the container a preservative atmosphere at a pressure greater than ambient atmospheric pressure while venting the interior of the container to atmosphere, sealing the container, introducing further preservative atmosphere until the container is pressurised to the desired degree, and leaving the media within the container under pressure of the preservative atmosphere for the desired storage period.
While clearly it is possible to use and adapt a variety of known types of storage container for such purposes, it is preferred and substantially more efficient to use for such storage a container which is especially adapted for the purpose. Accordingly, in a second aspect, the present invention provides a media storage container comprising at least two relatively movable parts, the parts being movable from a position in which a medium to be preserved may be put into or extracted from the container and a position in which the parts form a closed sealed container about the medium, an injection valve enabling the injection of a protective atmosphere into the interior of the container under pressure, venting means enabling the escape of atmosphere from within the container when the protective atmosphere is injected into the container at a pressure higher than the ambient atmosphere outside the container, and means for sealing the container while continuing to inject protective atmosphere to allow the inside of the container to be pressurised.
The venting means may be an exhaust valve located on a part of the container or, for example, may be constituted by so configuring the parts of the container that they may be loosely held together without sealing enabling atmosphere to be exhausted from the container, moving past the seal, whereafter the container parts may be finally moved into a sealing position.
The exhaust valve, if present, may be one which opens automatically in response to a given excess of pressure between the interior and exterior of the container when it is closed, which will act automatically to prevent over-pressurisation of the interior, or it may be operable between open and closed positions. Alternatively, over-pressurisation may be avoided by feeding the protective atmosphere into the closed container via a pressure reduction valve of known type, e.g. from such pressure gas cylinder via a valve which, once the back pressure reaches a predetermined level, emits to atmosphere.
Preferably, the container includes means for indicating the pressure inside the container relative to the pressure on the exterior. Such means may be used to check, e.g. that a slightly pressurised container has not become de-pressurised relative to ambient atmosphere. Most preferably, the container has incorporated within it a visually observable indicator giving a clear indication distinguishing between three conditions: the pressure inside the container is substantially equal to the atmospheric pressure outside it; the pressure within the container is within a desired range of pressure above ambient pressure; or the pressure within the container is too high. The advantage of a three-state indicator of this type is that the user of the container can see easily if the container is over-pressurised or has leaked. Over-pressurisation is undesirable since it places excess demands on the seal which can lead to failure of the seal and the return of the pressure within the interior of the container to atmospheric and, because the seal has failed, exchange between the ambient atmosphere and the atmosphere within the container.
The container may be made from any convenient material and may be sealed by any convenient known sealing method or construction. The sealing method is preferably one which provides a seal which is easily broken and easily resealable, so that archived material may be retrieved as necessary, inspected or copied, and then returned to the container which is then purged, resealed and re-pressurised. Most preferably the container is in the form of a two-piece casing having mating surfaces provided with a low pressure resistant seal, e.g. formations on one or both surfaces to provide an integral seal or provision of a separate strip, o-ring or the like of soft elastomeric material. Such containers may be easily produced in very large numbers and to high dimensional tolerances. Both pieces of the casing may be made by moulding from plastics materials or, for example, the casing may consist of a plastics moulding defining the media storage cavity and a metal lid adapted to fit thereon.
Such casings differ from conventional audio or video tape reel or cassette cases in having a seal when closed and having an injection valve (and, if fitted, the pressure indicator). It is possible using the principles of the present invention to provide an archival container which is re-usable, resealable and easy to put media into or remove media from.
In the case of two-part plastics containers according to the present invention, these may consist of two separable portions or may consist, for example, of two portions hinged together via an integral or live hinge formed of the plastics material of the container itself. The interior of the container may be appropriately configured to receive one or more media units such as tape reels, tape cassettes, optically readable discs, films or the like. The media units may be placed directly in the interior of the container or, if desired, an interior tray may be provided for the reception of the media to be stored. Such a tray should not, of course, inhibit the purging of the atmosphere from the container during purging and venting. The configuration of the tray can vary (for use with the same external container) to enable different media to be securely stored therein. This is of particular value in reducing the numbers of shapes and sizes which the outer container needs to be made in relative to the very wide variety of shapes and sizes of media to be stored. For example, the same container may be provided with an interior tray for the reception of a single large size tape cassette or an alternative form of tray for the reception of two or more smaller size tape cassettes.
The exterior of the container may be provided with conventional features, for example an integrally moulded handle or other gripping means enabling easier handling of the container, an area for receipt of an adhesive label, on which an inscription denoting the content of the container may be made, barcoding or a pair of loops for fixing a security tag to render unauthorised opening of the container detectable.
In one preferred construction, the container may be configured to be held in closed condition by a movable catch which, when the container is closed and the catch engaged, covers access to an exhaust valve. This prevents, in the case of a manually operated exhaust valve, inadvertent operation with consequent loss of pressure. Other catches may be provided to enhance holding the container closed. They may be dimensioned so that as they are engaged, they aid the seal between the two parts.
In an alternative preferred construction, the container may consist of two separable portions, the one defining a cavity for the receipt of the medium to be preserved and the other constituting a lid which may itself be hollow to surround, when the container is closed, part of the medium, and which may be engaged with the cavity defining portion in two distinct positions, in one of which it is a loose, i.e. unsealed, fit on the cavity defining portion and in the other of which it is sealed tightly thereto. This may be achieved conveniently by providing that the lid and cavity defining portion have co-acting formations on them constituting a cam closure mechanism. Most preferably, the lid or a part thereof may be slid between a position in which the lid is sealed to the remainder of the container and an intermediate loose fitting position, in both of which positions the camming means are inter-engaged one with another, and an end position in which the camming means are disengaged from one another and the lid member may be removed from the cavity defining member.
The preservative atmosphere injected into the container to purge it and, following its closure and sealing to form the preservative atmosphere may be chosen from a wide variety of possibilities. However, for reasons of safety, simplicity and economy, the preferred atmosphere is of nitrogen. The inlet valve may have an appropriate fitting to enable nitrogen to be injected using suitable apparatus, for example a pressurised canister or conventional gas cylinder, conveniently via a pressure-reduction valve of known type to avoid over-pressurising the container.